This is one of the fundamental benefits of the Delphix Engine. At the time of provisioning, the storage cost of any VDB is nearly zero since the entire content of the VDB at that time matches the source at some state in time so every single block points back to the source.
Blocks only diverge when write IO ops make changes that cause the VDB to differ from the source.
Consider the following:
dSource = 1TiB
You provision a VDB off of the dSource
The size of the VDB is 1TiB but on disk it is 0 (excepting the small overhead for filesystem pointers which are negligable)
You make 100MiB worth of changes to the VDB.
The size on disk is now just 100MiB
Alternatively, if it was used for read-only IO ops, it would stay at nearly 0.